The power loss in long downhole electrical transfer cables in connection with the recovery of petroleum may be considerable, and the maximum allowed transfer voltage is set by official regulations. Further, both downhole generators and electromotors must be limited in size because of the limited diameter of the wellbore. Downhole electromotors therefore have limited power. Because of that, it may be a challenge to provide sufficient forces for carrying out various operations downhole. Downhole actuators may use ball screws or the like to convert rotation into linear motion, wherein a shaft rotates inside a sleeve. The shaft and the sleeve are formed with complementary threads, and thus, when the shaft is rotated, the sleeve is moved along the shaft. The friction loss in such a ball screw can be considerable, and the ball screw may be subjected to great concentrated loads by great axial loads on the actuator.
The patent document EP 2175100 A1 discloses an apparatus for downhole directional drilling. The apparatus uses a linear, electrically operated actuator.
The patent document US 2012004271 A1 discloses a mechanical actuator for use in wind mills, among other things. The actuator uses roller screws to convert rotation into linear movements.